Reflow is widely known as a technique for mounting electronic components on a printed wiring board. In a reflow method, solder paste is applied to electrodes of a printed wiring board by printing through a metal mask, then electronic components are mounted at predetermined positions on the printed wiring board, and all the components are soldered at one time. For soldering, the printed wiring board on which the electronic components have been mounted is heated in a furnace.
Since the reflow method allows components of varying sizes to be bonded at one time, it is superior in productivity and has a great advantage in cost efficiency. In a process using the reflow method, it is desirable not to vary heat capacity among electronic components.
Small components include, for example, chip components, IC's, electric field capacitors. Large components include, for example, QFP's (quad flat packages) and BGA's (ball grid arrays). Small components generally have small heat capacities, and accordingly their temperatures are easily increased. On the other hand, large components generally have large heat capacities, and accordingly their temperatures are not easily increased.
The reflow method uses solder. As the use of lead is reduced, the use of lead-free solder is being spread. In general, lead-free solder has a higher melting point than eutectic solder. Accordingly, the reflow temperature of lead-free solder tends to be high. Reflow temperature is generally set at a temperature 10° C. to 30° C. higher than the melting point of solder so that the solder may melt and achieve bonding in a short time.
In use of a lead-free solder, if the heating temperature is increased to a temperature at which component having large heat capacities may be bonded, components having small capacities are thermally damaged by rapid temperature increase. If the heating temperature is reduced to avoid such thermal damage, however, the joints may not be heated sufficiently to melt, wet and spread the lead-free solder.
In order to prevent the thermal damage of small components, Japanese Laid-open Patent Publication No. 2001-348488 discusses a technique for protecting the components with a compact of a thermally conductive resin composition containing an epoxy resin and an inorganic filler. Unfortunately, the thermally conductive resin composition is used in a state where it is integrated with a component. Accordingly, the volume of the electronic component is undesirably increased after being mounted.